The present invention relates to a magnetoresistive effect element, a magnetic memory element, a magnetic memory device and manufacturing methods thereof, and particularly to a magnetoresistive effect element having stable magnetic characteristics, a magnetic memory element operable as a magnetic nonvolatile memory element, a magnetic memory device composed of the magnetic memory element and manufacturing methods thereof.
As information communication devices, in particular, personal small devices such as personal digital assistants are making great spread, elements such as memories and logics comprising information communication device are requested to have higher performance such as higher integration degree, higher operation speed and lower power consumption. In particular, technologies for making nonvolatile memories become higher in density and larger in storage capacity are progressively increasing their importance as technologies for replacing hard disk and optical disc that cannot be essentially miniaturized because they have movable portions.
As nonvolatile memories, there may be enumerated flash memories using semiconductors and FRAM (Ferro electric Random Access Memory) using ferroelectric material and the like.
However, the flash memory encounters with a drawback that its write speed is as slow as the microsecond order.
On the other hand, it is pointed out that the FRAM has a problem in which it cannot be rewritten so many times.
A magnetic memory device called an MRAM (Magnetic Random Access Memory), (e.g. see “Wang et al., IEEE Trans. Magn. 33 (1977), 4498), receives a remarkable attention as a nonvolatile memory which can overcome these drawbacks.
Since this MRAM is simple in structure, it can easily be integrated at a higher integration degree. Moreover, since it is able to memorize information based upon the rotation of magnetic moment, it can be rewritten so many times. It is also expected that the access time of this magnetic random access memory will be very high, and it was already confirmed that it can be operated at the access time of nanosecond order.
A magnetoresistive effect element for use with this MRAM, in particular, a tunnel magnetoresistive (Tunnel Magnetoresistance) element is fundamentally composed of a lamination layer structure of a ferromagnetic tunnel junction of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer.
This element generates magnetoresistive effect in response to a relative angle between the magnetizations of the two magnetic layers when an external magnetic field is applied to the ferromagnetic layers under the condition in which a constant current is flowing through the ferromagnetic layers. To be concrete, when the magnetization directions of the two magnetic layers are anti-parallel to each other, a resistance value is maximized. When they are parallel to each other, a resistance value is minimized. Function of memory element can be demonstrated by creating the anti-parallel state and the parallel state with application of the external magnetic field when the magnetization direction of one ferromagnetic layer is inverted.
This resistance changing ratio is expressed by the following equation (1) where P1, P2 represent spin polarizabilities of the respective magnetic layers.2P1P2/(1−P1P2)  (1)The resistance changing ratio increases as the respective spin polarizabilities increase. With respect to a relationship between materials for use with ferromagnetic layers and this resistance changing ratio, ferromagnetic chemical elements of Fe group such as Fe, Co, Ni and alloys of three kinds thereof have already been reported so far.
With respect to writing of information in the MRAM, in order to store information in the selected memory element of the magnetic memory element, this magnetic random access memory is composed of a plurality of bit write lines, a plurality of word lines intersecting these bit write lines and TMR elements provided at crossing points between these bit write lines and word write lines as magnetic memory elements. Then, when information is written only in the element located at the selected crossing point of these bit write line and word write line by utilizing an asteroid characteristic (see Japanese laid-open patent application No. 10-116490, for example).
The bit write line and the word write line used in that case are made of conductive thin films such as Cu and Al which are generally used by semiconductor devices. In this case, when information is written in a magnetic memory element of which the inverted magnetic field for generating the above-mentioned inversion of the magnetization is 40 [Oe], for example, by the bit write line and the word write line, the bit write line and the word write line being 0.35 μm in width, a current of approximately 10 mA was required. In this case, assuming that the thickness of the write line be the same as the line width, then a current density required at that time becomes 8.0×10−6 A/cm2. There is then a risk that breaking of wire will be caused by electromigration.
Accordingly, from a standpoint of the occurrence of electromigration and further in view of a problem of heat generated by the recording current and from a standpoint of decreasing a power consumption, this recording current has to be decreased.
As a means for decreasing the recording current, there is enumerated a method of decreasing an inverted magnetic field of a TMR element, i.e., coercive force.
The coercive force of this TMR element is determined based upon the size, shape of the element, the film arrangement and the selection of the materials. However, although it is desired that the size of the element should be miniaturized for the purpose of increasing a recording density of the MRAM, for example, the coercive force of the element tends to increase due to the miniaturization of the element. Therefore, the decrease of the coercive force should be attained from the material standpoint.
If the magnetic characteristic of the magnetic memory element is dispersed at every element and the magnetic characteristic is dispersed when the same element is used repeatedly, the selective writing using the asteroid characteristic becomes difficult.
Therefore, the magnetic memory element is requested to have a magnetic characteristic by which an ideal asteroid curve can be drawn. In order to draw the ideal asteroid curve, an R—H (resistance-magnetic field) curve obtained when TMR is measured should not have noises such as a Barkhausen noise, a rectangle property of a waveform should be excellent, the magnetization state should be stable and the dispersion of the coercive force Hc should be small.
Information may be read out from the magnetic memory element as follows. When magnetic moments of one ferromagnetic layer and the other ferromagnetic layer across the tunnel barrier layer are anti-parallel to each other and a resistance value is high, this state is referred to as a “1”, for example. Conversely, when the respective magnetic moments are parallel to each other and a resistance value is low, this state is referred to as a “0”. Then, information is read out from the magnetic memory element based upon a difference current obtained at a constant bias voltage or a difference voltage obtained at a constant bias current in these “1” and “0” states.
Accordingly, when resistance dispersions between the elements are identical to each other, a higher TMR ratio (Rmax−Rmin) Rmin where a resistance value Rmin represents the low resistance state and a resistance value Rmax represents the high resistance state) is advantageous and hence a magnetic memory device that can operate at a high speed, having a high integration degree and having a low error rate can be realized.
A magnetic memory element having a fundamental structure of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer has a bias voltage dependence of TMR ratio., and it is known that the TMR ratio decreases as the bias voltage increases.
When information is read out from the element based upon the difference current or the difference voltage, it is known that the TMR ratio takes the maximum value of the read signal at a voltage (Vh) which decreases by half depending upon the bias voltage dependence. Accordingly, a smaller bias voltage dependence is effective for decreasing read error in the MRAM.
Therefore, the TMR element for use with the MRAM should satisfy the above-mentioned write characteristic requirements and the above-mentioned read characteristic requirements at the same time.
However, when the magnetization free layer is generally made of a magnetic material composed of only ferromagnetic transition metal chemical elements of Co, Fe, Ni, if the alloy compositions by which the spin polarizabilities shown by P1 and P2 in the aforementioned equation (1) are increased are selected, then the coercive force Hc tends to increase.
For example, when the magnetization free layer is made of Co75Fe25 (atomic %) alloy and the like, although a TMR ratio having large spin polarizabilities and which is greater than 40% can be maintained, it is unavoidable that the coercive force also increases. But instead, when the magnetization free layer is made of an Ni80Fe20 (atomic %) alloy which is referred to as a “permalloy” and the like, although the spin polarizabilities are small as compared with the case in which the magnetization free layer is made of the Co75Fe25 (atomic %) alloy so that the TMR ratio is lowered up to about 33%.
Further, when the magnetization free layer is made of a Co90 Fe10 (atomic %) alloy, although a TMR ratio of about 37% can be obtained and its coercive force can become an intermediate value between the above-mentioned Co75Fe25 (atomic %) alloy and Ni89Fe20 (atomic %) alloy, this magnetization free layer is inferior in rectangle property of an R—H loop and an asteroid characteristic by which information can be written in the element cannot be obtained.